The present invention relates to methods and apparatus for use in connection with the reduction of sodium sulfate to sodium sulfide.
In particular, the present invention relates to the use of such methods and apparatus in connection with recovery of sulfuric acid and sodium hydroxide in the manufacture of viscose fibers.
It is already known to heat sodium sulfate up to the region of its softening point by way of a carbon-containing reducing medium, so that sodium sulfide will result while at the same time carbon monoxide is formed. This process in general is carried out in a discontinuous manner and requires not only the separation of sodium sulfate dust but also the milling of sodium sulfate and the homogeneous mixing thereof with the reducing medium. It is also known to bring about the reduction of Glauber's salt while utilizing iron salt catalysts.
One possible important application of the present invention is in the viscose fiber industry where it is required to recover from accumulating waste chemicals fresh chemicals necessary for the processes carried out in the industry. During the manufacture of viscose fibers large amounts of sodium sulfate accumulate, and only a relatively small part of the accumulated sodium sulfate can be sold. At the same time, the manufacturing operation requires purchase of large amounts of sodium hydroxide as well as sulfur. The sodium sulfate which is not sold is delivered with waste water to streams and the like. The part of the sodium sulfate which is sold is used as a filler for washing mediums and also will in a finely divided form be returned with waste water to streams and the like. In order to protect the environment it is of course necessary to limit the use of sodium sulfate and to forbid the introduction thereof into streams or other bodies of water.